<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<!--<input type="button" value="加" onclick="f4()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f1()">
<input type="button" value="除" onclick="f3()">-->
<div>
    结果:<span></span>
</div>



<script>
    //2
    function f(x){
        let i = document.querySelector("#i1");
        let e = document.querySelector("#i2");
        let s = document.querySelector("span");
        if(isNaN(i.value,e.value)){
            s.innerText="输入错误!"
            return;
        }
        switch (x){
            case 1://加
                s.innerText = parseFloat(i.value)+parseFloat(e.value);
            break;
            case 2://减
                s.innerText = i.value-e.value;
                break;
            case 3://乘
                s.innerText = i.value*e.value;
                break;
            case 4://除
                s.innerText = i.value/e.value;
                break;
        }
    }
    //1
  /*  function f1(){
        let i = document.querySelector("#i1");
        let e = document.querySelector("#i2");
        let s = document.querySelector("span");

            if(isNaN(i.value,e.value)){
                s.innerText="输入错误!"
            return;
        }
        s.innerText = i.value*e.value;
    }
    function f2(){
        let i = document.querySelector("#i1");
        let e = document.querySelector("#i2");
        let s = document.querySelector("span");

        if(isNaN(i.value) || isNaN(e.value)) {
            s.innerText = "输入错误!"
            return;
        }
        s.innerText = i.value-e.value;
    }
    function f3(){
        let i = document.querySelector("#i1");
        let e = document.querySelector("#i2");
        let s = document.querySelector("span");

        if(isNaN(i.value) || isNaN(e.value)) {
            s.innerText = "输入错误!"
            return;
        }
        s.innerText = i.value/e.value;
    }
    function f4(){
        let i = document.querySelector("#i1");
        let e = document.querySelector("#i2");
        let s = document.querySelector("span");

        if(isNaN(i.value) || isNaN(e.value)) {
            s.innerText = "输入错误!"
            return;
        }
        s.innerText = parseFloat(i.value)+parseFloat(e.value);
    }*/
</script>
</body>
</html>